Turbine vane platforms in a single stage of a gas turbine engine are sealed vane-to-vane with featherseals which are retained in essentially smooth sided slots. Featherseals generally comprise thin, high temperature resistant metal strips placed between adjacent vane platforms in corresponding slots, with the ends of the slots blocked to ensure seal retention (see FIG. 6). While the featherseals must be thin enough to bend and seal during radial vane movement, it is essential that the seal edges continuously engage an essentially smooth surface to prevent gas seepage through gaps between the seal edge and the contacted slot wall. Such gas seepage between vane stages reduces overall turbine efficiency.
Featherseal slots are very narrow, on the order of 0.02-0.30 inches wide, with general practice in the industry to provide straight slots in the vane platforms utilizing conventional machining techniques. However, most vane platforms have curved end sections for aerodynamic efficiency. Consequently, these curved end sections must include additional metal to provide adequate material for including a straight slot, which adds to engine weight, reducing the thrust to weight ratio. Such a slot is illustrated by the phantom lines on FIG. 4. In addition to the additional metal and weight added to the vane, the slot, since it does not follow the optimal aerodynamic contour, is more prone to leakage, thereby reducing engine efficiency. While many turbine vane designs can accommodate the additional metal requirement, advanced vane designs, utilizing a relatively flat vane profile, cannot. Consequently, these advanced vanes have not been deemed practical as no method existed for providing a good vane-to-vane seal.
Producing curved slots using conventional techniques is impractical. As a conventional "friable" wheel removes metal, it breaks down and requires frequent truing and dressing to maintain its cutting power, particularly when grinding superalloy material. The wheel diameter and width consequently vary during the cut, producing a relatively irregular slot wall. In addition, the wheel must be replaced after cutting one or two slots. Attempts have also been made to provide curved featherseal slots utilizing electrical discharge machining (EDM). However, an EDM electrode does not remain sufficiently flat to produce a smooth slot wall with an air-sealable surface. Consequently, the search continues for a method and apparatus for providing relatively reproducible, curved featherseal slots in airfoil articles.